Add CompletenessChecking for Fetch/Push services

[tomcoldrick-ct]

Currently we naïvely assume that the blobs referenced in requests/responses exist in CAS without checking. We should add something analogous to the CompletenessCheckingBlobAccess to ensure that blobs we reference actually exist.

One thing to be careful of while implementing this is that PushBlob and PushDirectory requests contain an additional pair of digest lists which we will also need to ensure exist. Given this, we'll need to add an extra field (or two, if we want to distinguish between Blob/Directory) to the Asset proto to store these digests so we can check at push and fetch time.

Implementing #3 will give us this for free. Note also that the lack of GetTree in bb-storage along with the RAAPI's Directory based definitions make this difficult to implement efficiently.

[Qinusty]

Directory definition issue for reference bazelbuild/remote-apis#165.

Although if we're storing these in the AC as ActionResults (#3), it's possible that the OutputDirectory tree_digest will give us a means of completeness checking against a tree.

However building this tree for storing ActionResults on PushDirectory will lead to some GetTree functionality being required (or something similar written locally without the API call).

[tomcoldrick-ct]

Although if we're storing these in the AC as ActionResults (#3), it's possible that the OutputDirectory tree_digest will give us a means of completeness checking against a tree.

When doing this with Pushed blobs we will need to ensure we reference the digests given in the references_{blobs,directories} fields in the ActionResult to get the desired completeness checking, I believe.